An image projector such as an augmented reality projector changes the appearance of a physical object by projecting an image onto the object. Patterns, colours and visible textures are super-imposed onto the surface of the object to make the object look different or to provide information on the surface.
Augmented reality is useful for ensuring success in some medical procedures such as microsurgical or cosmetic procedures, even surgery on delicate human arts such as the human brain and so on. For example, a medical professional may find it difficult to appreciate the internal structure of a human part right underneath its skin just by looking at the surface of the part. The colour, glare and distribution of ambient light may smooth out and obscure superficial textures and reliefs. Thus, determining the exact locations of under-skin or subcutaneous parts such as major blood vessels can be difficult even in a well-lit environment and more so if the blood vessels lie under a significant deposit of subcutaneous fat. An augmented reality projector capable of differentiating subcutaneous structure may be used to mitigate this difficulty by enhancing visual contrast, for example, between subcutaneous blood vessels and surrounding tissues. The augmented reality projector for such medical applications may comprise an infrared imaging element. Blood protein in subcutaneous blood vessels absorbs near-infrared wavelengths while the surrounding tissues tend to reflect them. Thus, a negative infrared image can be captured with dark parts showing blood vessels in the human part and illuminated parts showing the surrounding tissues. The augmented reality projector then generates a positive image in visible colours based on the infrared image, which is projected onto the same human part to highlight the locations of blood vessels under the skin. For better indication of the blood vessels, the projected image may have been treated to increase image contrast, or to enhance edge detection and so on.
However, the usefulness of this technology is limited because currently available augmented reality projectors cannot achieve good field-of-view match between the projected image and the object; they are unable to provide faithful image size reproduction and cannot provide sufficient precision in coinciding the image onto the object due to angular mismatch between the infrared imaging element and the image projector. Digital signal processing techniques have been the option of choice to overcome this mismatch instead of the relatively tedious option of designing an optical system to do so. The infrared image is processed digitally to adjust the size and angle of the projected image to fit object. However, such digital signal processing techniques have been found inadequate for achieving highly precise field-of-view match, possibly because there are simply too many variables to calculate. Furthermore, such digital signal processing techniques requires high processing power and tends to compete for computing resources in a multi-tasking environment. Thus, an augmented reality projector is often unable to refresh the projected image smoothly and in real time. This limits the technology from being applied to augment the appearance of moving objects.
Moreover, augmented reality projectors tend to have an infrared light source to supply infrared for illuminating objects such as the human part. However, the luminance distribution of the projected image and that of the infrared are not the same, making the image look less integral on the object.
Therefore, it is desirable to provide an augmented reality projector which can give a good field-of-view match between the image and the object, and by which the image may be updated sufficiently quickly to follow the changes of a moving object.